So-called “hybrid modalities”, such as, for example, PET/CT, SPECT/CT, MR/PET and MR/SPECT, have recently been becoming increasingly important in medical imaging. In this context,    PET is positron emission tomography,    CT is computed tomography,    SPECT is single photon emission computed tomography,    MR is magnetic resonance tomography, and    NM is nuclear medical imaging.
Combining a modality with a high spatial resolution (preferably MR or CT) and a modality with high sensitivity (preferably nuclear medicine such as, for example, SPECT or PET, abbreviated as NM in the following) is advantageous in these combinations.
Movement unsharpness, which has a particularly strong effect on the heart, which beats cyclically during the examination and thus has considerable movement unsharpness, is a particular problem in the case of PET and SPECT, making it very difficult to assign the PET/SPECT signal to particular regions of the heart. This removes the advantage of the high spatial resolution of the combined modality. This problem is particularly unfavorable in the case of examining patients with heart diseases, who have an irregular or very high heart rate.
FIG. 1 clarifies the problem. Reference symbol 1 denotes a heart. Reference symbols 2 and 3 denote the heart wall during systole or diastole. The reference symbols V and V′ in each case denotes a particular voxel of the heart wall during the systole or diastole. From FIG. 1, it is clear that the particular voxel undergoes a transition V′-V between diastole and systole during a cardiac cycle, leading to the described movement unsharpness.
The prior art discloses so-called gating techniques, which are based on ECG signals. For this purpose, ECG electrodes are applied to the patient. An algorithm identifies R waves in the ECG signal and uses these to determine the cardiac phases, the R wave being the first positive deflection in the QRS complex. Subsequently, use is made only of the measurement data which fall into a particular cardiac phase (for example, the diastole). Up until now, movement artifacts have been satisfactorily compensated by this method.
However, these methods still have a number of disadvantages. Applying ECG electrodes is an additional, time-consuming procedural step. In the case of MR, the electromagnetic fields interfere with the ECG electrodes, requiring the use of special ECG systems. Nevertheless, it is not always possible to receive a signal of adequate quality.
Due to the discarding of data, the measuring time is only partly used, and thus increases accordingly.
In the case of patients with previous cardiac disorders, the ECG signal is often pathologically changed, so that the algorithm for automatically detecting the R waves does not function.
DE 10 2005 023 907 A1 discloses a known method, which enables improvement with regard to spatial blurring when displaying PET data.